University of Maryland, College Park, Md. -- Library of American Broadcasting: BMI Collection[X163]

This December 1885 letter from Edison to George F. Barker, professor of physics at the University of Pennsylvania, contains a drawing and description of a device for producing mechanical vibrations. The letter was written in response to Barker's request for a description of the apparatus, which generated rapid and intense vibrations similar to those "produced by a shock machine upon the system."